The present invention concerns a semi-submersible platform for offshore oil field operation and a method of installing such a platform. The invention more particularly concerns a platform of the above kind which comprises a) a buoyant sub-substructure consisting of a base and a plurality of columns upstanding from said base, b) a buoyant deck-hull mounted on said column and c) means for ballasting and deballasting at least said base of said sub-structure.
At present there is increasing interest in operating offshore oil fields at great depths, from several hundred meters to 1 500 meters or more, for example, from platforms carrying on their surface the equipment necessary for drilling, production of and possibly storing petroleum products extracted from the field. With fields at these depths it is not possible to use gravity platforms resting on the sea floor. So-called "semi-submersible" platforms can be used, floating on the surface of the water and moored by taut anchor lines or by catenary anchor lines. The use of the latter type of anchorage can be associated with sizing a deep draught platform of the above kind to reduce heave due to swell and consequently facilitates the installation of well heads at the surface.
Because of its deep draught, a platform of the above kind is conventionally constructed in deep water ports or other coastal sites, such as are found in Scotland and in the fjords of Norway in particular. The use of such a deep water site is necessary in particular for installing the top deck of the platform. The platform is then towed out to its final site and anchored there.
Alternatively, if only a shallow water marine site is available, the deck must be installed at the final offshore operation site, which necessitates the use of substantial and costly plant such as powerful floating cranes.
Also known in themselves are semi-submersible platforms of the type described in the preamble to this description in which, after the deck and the sub-structure are constructed, the former is mounted on the latter and the resulting combination is launched so that it can be towed to the final installation site with a shallow draught. The base of the sub-structure then floats on the surface of the water and the deck rests on the base, at the foot of the column or columns upstanding vertically from the base. On arrival on site the sub-structure is lowered into the water, causing the structure to float on the deck. The sub-structure is lowered using a rack and pinion type mechanism, for example, this motion being combined with appropriate ballasting of the sub-structure. The lowering of the base of the sub-structure is stopped at a predetermined required depth, for example in the order of 100 m. Relying on the buoyancy of the sub-structure, the deck is raised to the top of the columns using the same mechanism, to a height that provides a convenient freeboard between the deck and the surface of the water. Other procedures for installing a platform of the above kind have been proposed but they comprise the requirement for a mechanism for raising the deck at the final installation site and, of course, means for locking the deck to its support columns at its final position.
This mechanism must be able to assure precise lateral guidance of the deck as it is raised and to withstand very high vertical loads due to the important weight of the deck. As a result these mechanisms account for a significant proportion of the cost and the reliability of the platform.
The means used to immobilize the deck on the columns in its final position conventionally consist in casting a concrete joint in the case of a concrete base or using welds in the case of a metal structure. These locking means are therefore non-reversible, i.e. they cannot be unlocked, which rules out or complicates replacement of the deck or dismantling of the platform, for example at the end of its service life.
Reversible locking means using "combs" meshing with the teeth of the rack of the deck-lifting mechanism are known in themselves but this solution obviously requires the presence of a mechanism which, as already mentioned, significantly increases the cost of constructing a platform of the above kind.